Yea, what he said.
I used Diplomat springs.
Head's up. I'm sure I read in Shepherd's chasis book that to get the best handling Chrysler designed springs to be flat at ride load.
(AARs and T/As excepted. They had a slight arch to clear the side pipes.)
Some day, I'm going to go through the book again and be sure I haven't made this up.
Also, If you make your rear end stiffer you will increase tendency to oversteer.